It's coming to Mac, Linux, and Windows. Says so right on the Kickstarter page. They're using the <a href="http://www.unity3d.com">Unity engine</a>, which can cross-compile to a plethora of platforms.
Yeah, I saw that on Kickstarter! On the Rooster feed they kept saying PC and I was getting upset. But then I read that on KS and immediately made my Pledge! Heck, you wouldn't think that Lord British woulda forgotten his Apple roots!!!
Garriott explains here - http://sota.ultimacodex.com/interview-with-richard-garriott/ - that he uses "PC" in a somewhat broad sense of the term, and includes Linux and Mac in his definition thereof.